fwrite

 

C

Language

Headers

Type support

Program utilities

Variadic function support

Error handling

Dynamic memory management

Date and time utilities

Strings library

Algorithms

Numerics

Input/output support

Localization support

Atomic operations (C11)

Thread support (C11)

Technical Specifications

 

File input/output

Functions
File access
fopenfopen_s (C11) freopenfreopen_s (C11) fwide (C95)	setbuf setvbuf fclose fflush
Direct input/output
fread
fwrite
Unformatted input/output
fgetc fgets fputc fputs getchar getsgets_s (until C11)(since C11) putchar puts ungetc	fgetwcgetwc (C95)(C95) fgetws (C95) fputwcputwc (C95)(C95) fputws (C95) getwchar (C95) putwchar (C95) ungetwc (C95)
Formatted input
scanffscanfsscanfscanf_sfscanf_ssscanf_s (C11)(C11)(C11) wscanffwscanfswscanfwscanf_sfwscanf_sswscanf_s (C95)(C95)(C95)(C11)(C11)(C11)	vscanfvfscanfvsscanfvscanf_svfscanf_svsscanf_s (C99)(C99)(C99)(C11)(C11)(C11) vwscanfvfwscanfvswscanfvwscanf_svfwscanf_svswscanf_s (C99)(C99)(C99)(C11)(C11)(C11)
Formatted output
printffprintfsprintfsnprintfprintf_sfprintf_ssprintf_ssnprintf_s (C99)(C11)(C11)(C11)(C11) wprintffwprintfswprintfwprintf_sfwprintf_sswprintf_ssnwprintf_s (C95)(C95)(C95)(C11)(C11)(C11)(C11)	vprintfvfprintfvsprintfvsnprintfvprintf_svfprintf_svsprintf_svsnprintf_s (C99)(C11)(C11)(C11)(C11) vwprintfvfwprintfvswprintfvwprintf_svfwprintf_svswprintf_svsnwprintf_s (C95)(C95)(C95)(C11)(C11)(C11)(C11)
File positioning
ftell
fgetpos
fseek
fsetpos
rewind
Error handling
clearerr
feof
ferror
perror
Operations on files
remove
rename
tmpfiletmpfile_s (C11)
tmpnamtmpnam_s (C11)

 

Defined in header <stdio.h>
size_t fwrite( const void *buffer, size_t size, size_t count,                FILE *stream );		(until C99)
size_t fwrite( const void *restrict buffer, size_t size, size_t count,                FILE *restrict stream );		(since C99)

Writes count of objects from the given array buffer to the output stream stream. The objects are written as if by reinterpreting each object as an array of unsigned char and calling fputc size times for each object to write those unsigned chars into stream, in order. The file position indicator for the stream is advanced by the number of characters written.

Parameters

buffer	-	pointer to the first object in the array to be written
size	-	size of each object
count	-	the number of the objects to be written
stream	-	pointer to the output stream

Return value

The number of objects written successfully, which may be less than count if an error occurs.

If size or count is zero, fwrite returns zero and performs no other action.

Example

Run this code

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
enum { SIZE = 5 };
int main(void)
{
    double a[SIZE] = {1, 2, 3, 4, 5};
    FILE *f1 = fopen("file.bin", "wb");
    assert(f1);
    size_t r1 = fwrite(a, sizeof a[0], SIZE, f1);
    printf("wrote %zu elements out of %d requested\n", r1,  SIZE);
    fclose(f1);
 
    double b[SIZE];
    FILE *f2 = fopen("file.bin", "rb");
    size_t r2 = fread(b, sizeof b[0], SIZE, f2);
    fclose(f2);
    printf("read back: ");
    for(size_t i = 0; i < r2; i++)
        printf("%f ", b[i]);
}

Output:

wrote 5 elements out of 5 requested
read back: 1.000000 2.000000 3.000000 4.000000 5.000000

References

• C11 standard (ISO/IEC 9899:2011):

• 7.21.8.2 The fwrite function (p: 335-336)

• C99 standard (ISO/IEC 9899:1999):

• 7.19.8.2 The fwrite function (p: 301-302)

• C89/C90 standard (ISO/IEC 9899:1990):

• 4.9.8.2 The fwrite function

See also

printffprintfsprintfsnprintfprintf_sfprintf_ssprintf_ssnprintf_s (C99)(C11)(C11)(C11)(C11)	prints formatted output to stdout, a file stream or a buffer (function)
fputs	writes a character string to a file stream (function)
fread	reads from a file (function)
C++ documentation for fwrite